Perched at the top of this week's singles chart, backed by a thrilling mechanical clank provided by urban producer Timbaland, Justin Timberlake has a peculiar announcement to make. "I'm bringing sexy back," he snarls, his voice fraught with electronic distortion. "You motherfuckers don't know how to act." This seems an odd thing to sing on two counts. For those unaware that sexy had gone anywhere in the first place, the initial claim seems presumptuous at best. For anyone who has sat through Timbo's performance as journalist Josh Pollack in the straight-to-DVD thriller Edison Force, the second claim seems suspiciously like the pot calling the kettle: look who's talking, Stanislavski.

But then, the interviews to promote his second solo album suggest that Timberlake has, of late, developed a curious way of expressing himself. "Music needs an enema," he told one journalist, stopping just short of calling FutureSex/LoveSounds the rectal bulb syringe of pop. Later, he outlined his plans for the future. Foremost among these was the fulfilment of a unique personal ambition: "I've got to kick myself in the balls."

You could argue all day about whether such an achievement is physically possible, but it's worth remembering that Timberlake has already pulled off one unfeasible feat. He has escaped the world of the manufactured boyband, enticing America's hippest, most innovative urban producers to set aside their prejudices, work their sonic magic and aid his quest to become a serious artist. Admittedly, this seems less amazing than it once did - in the years since his 2002 debut Justified, a raft of depressing releases have given the impression that America's hippest and most innovative urban producers would be willing to set aside their prejudices and work their sonic magic on a bag of ericaceous compost if its cheque contained enough zeros - but you can't deny that Timberlake's crossover has been the most successful of the lot. Justified sold 7m copies worldwide.

Initially, at least, FutureSex/ LoveSounds suggests that Timberlake's talk of innovation may be more than mere bluster. Timberlake has mentioned David Bowie as a musical touchstone. The cynic would say that's precisely the kind of name an ex-boyband member would drop to impress journalists already befuddled by talk of musical enemas and kicking oneself in the balls. But, improbably enough, you can definitely hear the influence of Bowie's 1974 album Diamond Dogs on FutureSex/LoveSounds' opening songs. It's in the claustrophobic atmosphere, the preponderance of weird vocal effects, the way tracks segue via abstract interludes or crash into each other without warning. The latter is hardly a new trick, but it still proves wildly effective, not least when SexyBack suddenly lurches out of the title track, to heart-racing effect. Elsewhere, LoveStoned dramatically and unexpectedly switches gear midway through: the sweaty, slap-bass funk vanishes, in favour of vigorously strummed, vaguely Sonic Youth-ish guitars, luscious strings and a gorgeous Kraftwerk-inspired counter-melody. My Love, meanwhile, is little short of astonishing, a twitching mass of rave synthesizers and agonisingly slow beats.

There are drawbacks, not least the lyrics. "Let's take a trip to Dubai," Timberlake croons seductively. "You know I want a piece of that pie." Dubai? Home of Jim Davidson, repressive authoritarianism, and forced hormone therapy for homosexuals? That sounds romantic. Meanwhile, What Goes Around Comes Around aims another kick at his most famous ex: "You're living a lie," he wails. "You got what you deserved." Four years ago, Cry Me a River's Britney-bashing had a certain gossipy compulsion. Now it seems needlessly sour, given she's the one whose career is in freefall and who appears to have married Jed Clampett by mistake, and he's the one who's boffing Cameron Diaz.

But the lyrics don't really matter, as long as there's endlessly shifting sonic fireworks on offer to distract your attention. FutureSex/LoveSounds' problems really begin when Timbaland abruptly packs up his box of production tricks and the album melts in a mass of gloop. A dreary central-casting pop ballad called Summer Love is enlivened only by another of Timberlake's peculiar announcements: "I'm sick and tired of trying to save the world," he cries. Sad news, especially for those of us who must have missed Timberlake's strenuous efforts on the world's behalf. There is Losing My Way, a dreadful little song about crack addiction. By its closer - boring ersatz southern soul overseen by Rick Rubin - FutureSex/LoveSounds feels like an entirely different and far less interesting album than the one you started listening to. It's as if Timberlake has suddenly lost confidence in his ability to irrigate pop's colon, and reverted to type, making precisely the kind of music you would expect a former boyband member to make.

So FutureSex/LoveSounds almost works: close, but no enema. Still, you wonder what he might come up with next time, if his nerve holds. The world awaits with baited breath the presumably unique sound of Timberlake kicking himself in the balls.